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  #21  
Old 06-15-2016, 05:01 PM
Marc Bourget Marc Bourget is offline
 
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I had to address this (no fins there) issue with another engine.

It's important to overall cooling performance that the cross-sectional area of the cooling passage above the no-fin area remains limited, close if not equal to the space between the fins.

I ended up making a punch and die and formed them on my press brake.

FWIW

mjb

Last edited by Marc Bourget : 06-15-2016 at 05:02 PM. Reason: added a hypen between no and fin
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  #22  
Old 06-15-2016, 05:36 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by Marc Bourget View Post
It's important to overall cooling performance that the cross-sectional area of the cooling passage above the no-fin area remains limited, close if not equal to the space between the fins.
Care to expand on that statement ?
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  #23  
Old 06-16-2016, 09:10 AM
Marc Bourget Marc Bourget is offline
 
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DanH asked: "Care to expand on that statement ?"

Sure,

1st, you want to ensure that adequate air mass (# of molecules) flows THROUGH the fins, as that's the only thing that carries heat away.

2nd, you want turbulent air in between the fins to provide more molecule-to-fin contact. No or not much heat transfer occurs via radiation.

3rd, Hopefully, engine designers were aware of Reynolds effect and spaced the fins consistent with the amount of mass necessary to cool the engine and sufficient to insure turbulence in the "hydraulic cylinder" or "cooling tube" (terms used in evaluating fin spacing).

4th, if you have a section that's oversized, more air than necessary flows through, and probably slower, to the point that maybe no turbulence generated, AND, it drops the pressure differential (to some degree, hopefully not too much) for the rest of the engine, reducing the mass that flows through the rest of the "system".

Less mass, less cooling. Significantly less mass can lead to much more loss of cooling than simple drop in mass (consistent with loss of turbulence in the "cooling tube")

Hope the above was clear and responded to your question.

FWIW

mjb
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  #24  
Old 06-16-2016, 02:52 PM
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DanH DanH is offline
 
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Not allowing air to bypass the fins is obvious enough.

Reynolds number is a bit more subtle...

http://www.vansairforce.com/communit...0&postcount=53

However, the question here revolves around sizing of an inlet upstream of an orifice. I'll state that it is not possible to "oversize" the section prior to the system orifice. For the most part, flow is controlled by the system orifice, not the approach to the orifice.
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  #25  
Old 06-16-2016, 05:12 PM
Marc Bourget Marc Bourget is offline
 
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Dan, Thanks for the link to the other post. I copied it for my compendium.

You said: "I'll state that it is not possible to "oversize" the section prior to the system orifice. For the most part, flow is controlled by the system orifice, not the approach to the orifice."

I'm in agreement with the first point, but I'm not "grokking" your reference to the "system orifice" as I interpret it as a counter point and my intent was to speak only about the orifice.

What am I missing?

mjb
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  #26  
Old 06-17-2016, 06:56 AM
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DanH DanH is offline
 
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Good discussion Marc. It triggers thinking.

As a practical matter, I don't a believe a TLAR approach will result in an entry ("D" below) that is too large, at least until it gets so large that airflow simply bypasses significant fin area. Although that would reduce heat transfer, actual flow would still be controlled by the orifice...the smallest area in the flow path. With a very large entry, the smallest area in the flow would be "E".

Set that point aside. A bit of thought says your original statement is valid, in so far that we should try to define some considered approach to sizing the bypass.

Below is a view looking in from the the starboard side. Cylinders #1 and #3 share flow area at C, after which that flow splits into areas A and B. Note that C is largely the depth of the fins on the exhaust side of Cyl #3. The classic problem on the intake (rear) side of #3 is that we have no adjacent cylinder to provide flow area.

So, we wish to find a dimension for D. A simple approach might be to approximate the percentage of C which is flow through B:

C(B/A)=D

The resulting dimension D would be a little larger than necessary, as the simple equation does not account for the cross section area of the fins in C. However, it might be a reasonable place to start.

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Last edited by DanH : 09-03-2018 at 06:53 AM.
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  #27  
Old 06-17-2016, 07:43 AM
Marc Bourget Marc Bourget is offline
 
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Will a D constructed to large lose turbulence?

My thought is, if the D dimension is about equal to fin spacing, it would work towards preserving turbulence and assist in maintaining pressure differential, overall.

On a different consideration, all else being approximately equal, will a 5.5" H2O Delta P preserve intra-cylinder velocity, irrespective of top vs. bottom absolute pressure readings?

Regards,

Marc
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  #28  
Old 06-17-2016, 08:15 AM
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Quote:
Originally Posted by Marc Bourget View Post
Will a D constructed to large lose turbulence?
Don't care....no fins there.

Quote:
My thought is, if the D dimension is about equal to fin spacing, it would work towards preserving turbulence and assist in maintaining pressure differential, overall.
Empirical evidence says no. I started with a very small slot and later went larger, which lowered cylinder temperature. Other reports mirror the trend. That suggests increased mass flow trumps velocity increase in the pinch area.

Quote:
On a different consideration, all else being approximately equal, will a 5.5" H2O Delta P preserve intra-cylinder velocity, irrespective of top vs. bottom absolute pressure readings?
DeltaP is the difference between top and bottom absolute pressures. And there's no magic in 5.5". The required deltaP (i.e. mass flow in lbs/sec) varies with density (i.e. altitude) and power setting.
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  #29  
Old 06-17-2016, 10:31 AM
BillL BillL is offline
 
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Quote:
Originally Posted by Marc Bourget View Post
Will a D constructed to large lose turbulence?

My thought is, if the D dimension is about equal to fin spacing, it would work towards preserving turbulence and assist in maintaining pressure differential, overall.

On a different consideration, all else being approximately equal, will a 5.5" H2O Delta P preserve intra-cylinder velocity, irrespective of top vs. bottom absolute pressure readings?

Regards,

Marc
1. It does not matter the initial flow is not turbulent as it will be 5 fin spacings into the channel.
2. Reynolds numbers show that the flow in the fins is well into turbulent flow even with low speed climbs (5.5" pressure drop). So, heat transfer coefficient is still very good. Cylinder and head fins.

My opening area at the pinch point (centerline of the head) is about 80% of the fin area down at E. Without the fin thickness it is not much, like 0.2". Being concern about how much flow there was, I made a nozzle with those dimensions and put it on the end of a vacuum on blow. At 10" h20 pressure it is amazing how much flow there is in that slot. I now use that as a vacuum nozzle in the shop.

I was concerned about this when making mods to my engine and did the calculations for parallel plates for the fins and a rectangular channel for the cylinders with a cylinder wrap. No problem what-so-ever. Apparently, the designers were aware.

Good discussion on this, understanding the relative effects of these things is important.

PS - all dimensions off a M1B parallel valve engine.
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  #30  
Old 06-17-2016, 11:19 AM
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rzbill rzbill is offline
 
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This discussion of fins reminds me of a point that was introduced in my Heat Transfer class. Just a point of idle interest...

We know pretty much all cooling fins are made straight and parallel.

The first pass with the thermodynamics math says they should be parabolic. Wide at the root and skinny at the top.

Yea, so...we were yawning.. (we studied all night y'know )

BUT, on deeper analysis, the graph plots had points that did not lie on the basic parabolic curve. By looking closer, it turns out that the parabolic curve was the centerline of what looked like a sine type wave. So.. the fin cross section really looked a lot like a Christmas tree

We were talking about this in 1980 about the time that Mandelbrot was doing his thing but it was not well known yet. Now I seriously wonder if the most efficient fin is really a fractal and we did not know it.

Of course it would be very difficult or impractical to manufacture.
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